The present invention generally relates to the field of optical instruments, and more particulaly to infrared sensitive imaging apparatus for performing continuous repeated or stepped transverse scans of a scene.
The use of infrared wavelengths is of known advantage when temperature information is required from a scene. Detectors that are sensitive to infrared wavelengths are necessary to convert the infrared energy to a form that is perceptible to an observer. If the radiation from a scene were projected on a mosiac of such detectors at a focal plane within the optical system and electrically sampled and formatted into a television (TV) type presentation a continuous picture of the scenes thermal image would result. In practice, mosiacs of detectors of the complexity necessary to yield high resolution imagery are not practical due to relatively low yields.
The alternatives to a complex detector mosaic based system are systems in which the field of view time-shares a single detector, a simple linear array of detectors, or a sparse mosiac, by the use of an opto-mechanical scanning arrangement. Masaics and linear (i.e., a row of detectors) arrays are of significant advantage in the application of TDI (time delay and integration) which utilizes the linear scan of the polygon scanner and electrical time delays within the signal processing to improve signal-to-noise ratios in the system to which it is applied. Time-sharing of a single detector by use of some form of area scanner is most efficient as far as detector utilization is concerned, but does not provide the sensitivity and resolution of a multiple detector system with a simple transverse scan. The latter system thus appears to offer the optimum compromise between complexity and sensitivity.
One past solution to the problem is described in The Infrared Handbook prepared by The Infrared Information and Analysis Center, Environmental Research Institute of Michigan and prepared for the Office of Naval Research, Department of the Navy, 1978, p.p. 10-17. This solution uses an internal polygon scanner. However, the optical path is folded in such a manner so as to produce a nonlinear "windshield wiper" scan pattern.
Another solution is described in U.S. Pat. No. 3,728,545, by Irving Abel. In this solution, the scanner is used in converging light, which makes the image appear to be on a curved surface. This field curvature can only be removed by the input optics for a narrow line of detectors. It is sometimes desirable to add detectors in the scan direction in order to increase sensitivity. This is known as time delay and integration (TDI). To do this with the aforementioned scanning system would require a focal plane matched to the field curvature caused by scanning in noncollimated light.
It is therefore the object of this invention to provide an improved infrared imaging system which incorporates a linear scan, whose compactness lies in its ability to include all optical elements in an in-line or coaxial configuration.